Inkjet printers have gained wide acceptance. These printers are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Inkjet printers produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet printers print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed nozzles attached to a printhead substrate that incorporates an array of ejection chambers which receive liquid ink from the ink reservoir. Each chamber has a thin-film ink ejection element, known as a inkjet ejection chamber ink ejection element, located opposite the nozzle so ink can collect between it and the nozzle. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the ink ejection element elements. When electric printing pulses heat the inkjet ejection chamber ink ejection element, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
In U.S. Pat. No. 5,442,384, entitled "Integrated Nozzle Member and TAB Circuit for Inkjet Printhead," a novel nozzle member for an inkjet print cartridge and method of forming the nozzle member are disclosed. A flexible circuit tape having conductive traces formed thereon has formed in it nozzles or orifices by Excimer laser ablation. The resulting flexible circuit having nozzles and conductive traces may then have mounted on it a substrate containing heating elements associated with each of the nozzles. The conductive traces formed on the back surface of the flexible circuit are then connected to the electrodes on the substrate and provide energization signals for the heating elements. A barrier layer, which may be a separate layer or formed in the nozzle member itself, includes ejection chambers, surrounding each orifice, and ink flow channels which provide fluid communication between a ink reservoir and the ejection chambers.
In U.S. Pat. No. 5,648,805, entitled "Adhesive Seal for an inkjet Printhead," a procedure for sealing an integrated nozzle and flexible or tape circuit to a print cartridge is disclosed. A nozzle member containing an array of nozzles has a substrate, having heater elements formed thereon, affixed to a back surface of the flexible circuit. Each orifice in the flexible circuit is associated with a single heating element formed on the substrate. The back surface of the flexible circuit extends beyond the outer edges of the substrate. Ink is supplied from an ink reservoir to the nozzles by a fluid channel within a barrier layer between the flexible circuit and the substrate. In either embodiment, the flexible circuit is adhesively sealed with respect to the print cartridge body by forming an ink seal, circumscribing the substrate, between the back surface of the flexible circuit and the body. This method and structure of providing a seal directly between a flexible circuit and an ink reservoir body has many advantages. Also, in U.S. Pat. No. 5,736,998, entitled "Inkjet Cartridge Design for Facilitating the Adhesive Sealing of a Printhead to an Ink Reservoir," and U.S. Pat. No. 5,852,460, entitled "Inkjet Print Cartridge Design to Decrease Deformation of the Printhead When Adhesively Sealing The Printhead to the Print Cartridge;" improved headland designs are disclosed which alleviate some of the above-mentioned problems.
Flexible circuit leads are bonded to pads or electrodes on the outer edges of the substrate. To enable this bonding, a window is created in the flexible circuit to allow a bonder thermode to apply force and temperature to the flexible circuit leads that are resting on the bond pads. After the leads have been bonded, an encapsulant is dispensed across the window to protect the exposed bond pad region from intrusion of ink or contamination.
By providing the nozzles in the flexible circuit itself, the shortcomings of conventional electroformed nozzle members are overcome. This integrated nozzle and tab circuit design is superior to the nozzle members for inkjet printheads formed of nickel and fabricated by lithographic electroforming processes.
In U.S. Pat. No. 5,450,114, entitled "Adhesive Seal for an Inkjet Printhead," a procedure for sealing an integrated nozzle and tab circuit to a print cartridge is disclosed. See also U.S. Pat. No. 5,736,958, entitled "Inkjet Cartridge Design for Facilitating the Adhesive Sealing of a Printhead to an Ink Reservoir."
However, the above designs did not address the problem of "dimples" being formed in the nozzle member caused by bending of the nozzle member due to the stresses created by the adhesive process of sealing the nozzle member to the print cartridge. This dimpling of the nozzle member creates nozzles which are skewed causing trajectory errors for the ejected ink droplets from the nozzles. When the TAB head assembly is scanned across a recording medium the ink trajectory errors will affect the location of printed dots and thus affect the quality of printing.
In a typical edge-feed inkjet printhead assembly 14 the flexible circuit 18 serves as both an nozzle member 16 and as a carrier of the conductor traces 36. There are several fundamental problems with this design approach. First, the conductor traces 36 must be protected with a cover layer 38 to prevent electrical shorting and corrosion and it is difficult to design a cover layer 38 and an adhesive 90 that is resistant to a wide range of inks and also has good adhesion to both the print cartridge 10 body and the flexible circuit 18 material. Second, most head-to-body adhesives de-laminate from the flexible circuit over time. When this occurs, inks can readily penetrate the cover layer adhesive and cause electrical shorting and corrosion. Third, on edge-feed print cartridges, there is an unprotected region between the edge of the cover layer and the substrate. Here, the conductor traces are susceptible to shorting and corrosion if ink penetrates between the structural adhesive and the flexible circuit. This gap is necessary because of the cover layer placement tolerances in manufacturing. The nozzle member material, thickness, and manufacturing processes cannot be optimized independently from that of the flexible circuit, even though each has very different functional requirements.
Accordingly, it would be advantageous to have an improved printhead design which reduces dimpling of the nozzle member and ink penetration and shorting of the substrate electrodes and flexible circuit leads.